Long term evolution (“LTE”) represents the project within the third generation partnership project (“3GPP”), with an aim to improve the Universal Mobile Telecommunications System (“UMTS”) standard. The goals are to support future requirements and include improved system capacity and coverage, reduced latency, higher peak data rates, and lower costs. The LTE project is not actually a standard, but results in an evolved release of the UMTS standard. Bandwidth is scalable in view of spectrum allocations, higher data rate requirements and deployment flexibility.
The LTE physical layer is designed to achieve higher data rates, and is facilitated by turbo coding/decoding, and higher order modulations, e.g., up to 64-QAM. LTE supports both frequency division duplexing (“FDD”) and time division duplexing (“TDD”) modes of operation.
Orthogonal frequency division multiple access (“OFDMA”) is used for the downlink (base station to mobile station), while single carrier frequency division multiple access (“SC-FDMA”) is used for the uplink (mobile station to base station). The main advantage of such schemes is that the channel response is flat over a sub-carrier even though the multi-path environment could be frequency selective over the entire bandwidth. This reduces the complexity involved in equalization, as simple single tap frequency domain equalizers can be used at the receiver. OFDMA allows LTE to achieve its goal of higher data rates, reduced latency and improved capacity/coverage, with reduced costs to the operator. By using multiple parallel data stream transmissions to a single terminal, data rate can be increased significantly. In a multi-path environment, such a multiple access scheme also provides opportunities for performance enhancing scheduling strategies.
Uplink virtual multiple-input multiple-output (“V-MIMO”), which uses spatial multiplexing (“SM”) across multiple mobile stations (“MS”), promises spectral efficiency gains without the need for additional transmit antennas at the MS. Traditional MIMO uses multiple antennas at a single MS. Multiple mobile stations, each using a single transmit antenna can be assigned the same physical resource to create a virtual MIMO transmitter. Further, V-MIMO as a feature is completely transparent to the mobile and requires no additional mobile processing. However, extracting performance gains using V-MIMO in a multi-cell environment is more challenging when compared to a single user MIMO scenario. The scheduler has to select users capable of sustaining a robust link in the presence of additional inter-layer interference and determine appropriate modulation coding scheme (“MCS”) downgrades. Scheduling of multiple mobiles on the same sub-carrier could result in additional inter-cell interference that will negatively affect low signal to interference ratio (“SINR”) users and coverage. Scheduling of users, user pairing & link adaptation are areas that need optimization to ensure that performance gains are realized without impacting coverage. In addition, interference increases due to pairing needs to be mitigated.
VMIMO pairing techniques used in a single cell environment provides low throughput gains when deployed in a multi-cell environment. Extracting performance gains in the uplink in a multi-cell layout requires appropriate pairing of MSs that could be based on several criteria. Current pairing algorithms focus on firstly creating a V-MIMO candidate list based on the uplink received SINR for each MS. MSs in this list are paired in a random fashion or based on channel metrics, such as Orthogonal Factors (“OF”). Current schemes do not provide any sector throughput gains, but could lead to a reduction of throughput or increased outage for cell-edge users.
Candidate list generation based on Uplink received SINR ensures that each paired MS will be capable of supporting at least the lowest modulation coding scheme (“MCS”) at a low frame error rate (“FER”) and yields gains when V-MIMO is deployed in hot-spots. However, it does not consider the amount of additional interference that is generated due to pairing when V-MIMO is deployed over several cells. The current pairing schemes can result in increased inter-cell interference and Interference over Thermal (“IoT”). The increased IoT reduces MIMO gains leading to an overall throughput loss & poor performance for cell-edge users.
Another solution uses higher uplink SINR thresholds to add UEs to the candidate list. However, the method does not work well because V-MIMO candidate list generation is based on uplink SINR threshold. Since uplink transmissions are power controlled, cell edge UEs may use a high transmit power to attain a high receive SINR. This practice allows such UEs to qualify for the candidate list, which when paired together, generate significant amounts of interference to neighboring cells. As this threshold is increased, very few UEs qualify for the candidate list. But, these UEs can create significant interference to neighboring sectors. Again, the overall interference increase is not mitigated and the SM gains are minimal because most UEs do not qualify for the candidate list.
Therefore, what is needed is a method and system for candidate list generation which maximizes the benefits of V-MIMO.